1. Technical Field
The present invention relates generally to radio frequency (RF) signal circuitry, and more particularly, to single band transmit-receive front-end integrated circuits for time domain duplex communications.
2. Related Art
Wireless communications systems find application in numerous contexts involving information transfer over long and short distances alike, and there exists a wide range of modalities suited to meet the particular needs of each. These systems include cellular telephones and two-way radios for distant voice communications, as well as shorter-range data networks for computer systems, among many others. Generally, wireless communications involve a radio frequency (RF) carrier signal that is variously modulated to represent data, and the modulation, transmission, receipt, and demodulation of the signal conform to a set of standards for coordination of the same. For wireless data networks, such standards include Wireless LAN (IEEE 802.11x), Bluetooth (IEEE 802.15.1), and ZigBee (IEEE 802.15.4), which are understood to be time domain duplex systems where a bi-directional link is emulated on a time-divided single communications channel.
A fundamental component of any wireless communications system is the transceiver, that is, the combined transmitter and receiver circuitry. The transceiver, with its digital baseband subsystem, encodes the digital data to a baseband signal and modulates the baseband signal with an RF carrier signal. The modulation utilized for WLAN, Bluetooth and ZigBee include orthogonal frequency division multiplexing (OFDM), quadrature phase shift keying (QPSK), and quadrature amplitude modulation (16QAM, 64QAM). Upon receipt, the transceiver down-converts the RF signal, demodulates the baseband signal, and decodes the digital data represented by the baseband signal. An antenna connected to the transceiver converts the electrical signal to electromagnetic waves, and vice versa.
Almost all conventional electronic devices with wireless communication capabilities implement more than one system or standard. For example, a cellular telephone may include a WLAN subsystem for high-speed data transfers, in addition to a Bluetooth subsystem for concurrent pairing with wireless headsets and the like. Such multi-modality wireless communications systems include a separate transceiver for each of the different subsystems, and each transceiver may include a dedicated transmit (TX) line and a dedicated receive (RX) line. In the alternative, however, the transceiver may have a combined TX/RX line. In most cases, the WLAN transceiver has separate TX and RX lines, while the Bluetooth transceiver has a combined TX/RX line. The transmit line and the receive line of all of the transceivers are tied to a single antenna.
RF circuitry such as the transceiver is produced as integrated circuits, typically with complementary metal-oxide semiconductor (CMOS) technology, due in part to the successes in miniaturization and cost reduction efforts. Small geometry CMOS devices have reduced current draw and require lower battery voltages, thus being suitable for portable applications that have substantial power consumption limitations. Wireless communication links must be reliable and have high data throughput over wide distances, necessitating higher power levels at the antenna stage. For instance, the aforementioned Wireless LAN typically requires power levels of up to and above 20 dBm.
Higher power output, in turn, requires higher current and voltage levels in the RF circuitry. Many CMOS devices are currently produced with a 0.18-micron process, with advanced systems utilizing 130 nm, 90 nm, 65 nm, and 45 nm processes. The resulting integrated circuits have operating voltages in the range of 1.8 v to lower than 1.2 v because of the reduced break down voltages of the semiconductor devices therein. Although current draw is typically not an issue because of the existence of simple solutions involving multiple active devices connected in parallel, +20 dBm power levels at 1.8 v have been difficult to achieve, particularly for signals having envelope variations, as is the case with OFDM, QPSK, QAM, and the like. Indeed, peak power may be 5 dB to 10 dB higher than average due to stringent linearity requirements for the transmitted signal, and the typical 1 dB gain compression (P1 dB) of the signal may reach 24 dBm to 27 dBm. Increasing current draw introduces several new issues including decreased efficiency because of a greater proportion of power being lost as heat, and decreased battery life. Furthermore, the impedance is lowered for the same power level with increased current. Considering that most RF circuits have a 50-Ohm impedance, the design of matching circuits for decreased impedance also becomes an issue, typically due to increased power losses.
Conventional WLAN transceivers typically do not generate sufficient power or have sufficient sensitivity necessary for reliable communications. Current integrated circuit transceiver devices have transmit power levels of below 0 dBm, though there are some devices that have power levels of about 10 dBm, which is still significantly less than the desired 20 dBm noted above. Accordingly, additional conditioning of the RF signal is necessary. With regard to Bluetooth transceivers, however, 0 dBm output at the antenna may be sufficient for class-3 operation, while 4 dBm may be sufficient for class-2 operation, so additional amplification may not be required.
The circuitry between the transceivers and the antenna is referred to as the front-end module, which includes a power amplifier for increased transmission power, and/or a low noise amplifier for increased reception sensitivity. Various filter circuits such as band pass filters may also be included to provide a clean transmission signal at the antenna, and/or to protect the reception circuitry from external blocking signals reaching the antenna. In order to rapidly switch between receive and transmit functions, and in order to prevent interference during the transitions between transmission and reception, the front-end module also typically includes an RF switch that is controlled by a general-purpose input/output line of the transceiver.
As noted above, conventional multi-modality wireless communications systems include multiple transceivers that are connected to a single antenna. WLAN, Bluetooth, and ZigBee are understood to share the same operating frequency band, that is, the industrial-scientific-medical (ISM) band of 2.4 GHz to 2.5 GHz, so simultaneous operation is not possible without substantial signal degradation. Accordingly, the operation of the two transceivers is scheduled according to predefined priority levels. In typical integrated systems with various combinations of WLAN, Bluetooth, ZigBee, or other time-domain duplex system, the RF switch is a single-pole, triple throw switch connecting the antenna to the input of the low noise amplifier for one of the transceivers, the output of the power amplifier for another one of the transceivers, or the combined input and output thereof. Further control over the operation of the power amplifier and the low noise amplifier may be possible with the enable output from the transceiver. The enable line may have varying voltages to control gain or setting the bias current of the transistors in the amplifier circuitry
Interrelated performance, fabrication, and cost issues have necessitated the fabrication of the RF switch on a different substrate than the substrate of the power amplifier and the low noise amplifier. Power amplifiers are typically fabricated on a gallium arsenide (GaAs) substrate, which is understood to provide high breakdown voltages and reliability. Other substrates such as silicon germanium (SiGe) may also be utilized. Furthermore, the power amplifier can utilize heterojunction bipolar transistors (HBT), metal-semiconductor field effect transistors (MESFET) or high electron mobility transistors (HEMT), with the HBT being the least costly to fabricate. Along these lines, the low noise amplifier may also be fabricated on a GaAs substrate with HBT transistors. However, because of high insertion loss or low isolation, an RF switch using HBT transistors suffers from poor performance characteristics.
Various solutions to the forgoing issues have been proposed. One involves a multi-die configuration in which the power amplifier and the low noise amplifier are fabricated on one die using HBT transistors, and the RF switch is fabricated on another die using, for example, HEMT transistors. Both of the dies are then encapsulated in a single package. The added costs associated with the GaAs substrate as compared to conventional silicon substrates, and the complex packaging process further elevates the cost of the front-end module fabricated in accordance therewith. Another proposal is directed to a composite GaAs substrate having both HBT and HEMT transistors for the power amplifier and the low noise amplifier, and the RF switch, respectively. Again, however, such integrated circuits are costly to manufacture. Yet another proposal is the use of a silicon substrate for the low noise amplifier, the power amplifier, and the RF switch. Because of poor isolation associated with silicon substrates, however, higher cost solutions such as silicon on insulator (SOI) may be used. These integrated circuits typically require a negative voltage generator, which results in a larger die for its bias circuitry. Additionally, spurious signals over a wide frequency range emitted by a charge pump for the negative voltage generator necessitates a physical separation thereof that further increases die size.
The RF switch thus represents a significant constraint on the design of transceiver front-ends. Accordingly, there is a need in the art for RF transmit/receive front-end circuits without conventional RF switches with sufficient transmitter output and receiver sensitivity for time-domain duplex applications.